1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process or an electrostatic recording process and in particular to an image forming apparatus in which an amount of toner formed on an image bearing member is controlled.
2. Related Background Art
FIG. 22 shows a color image forming apparatus of the electrophoto graphic process. The image forming process will be outlined. First, on a photosensitive drum 101, a toner image based on image information is formed. Then, the toner image on the photosensitive drum 101 is transferred to a transfer material adsorbed on a transfer drum 108. This transfer step is repeated by the times needed to form a desired image on the transfer material. Thereafter, the transfer material is separated from the transfer drum 108 and the toner image is fixed on the transfer material by a fixing device 107 and discharged from the apparatus outward.
Besides, as alternative example, FIG. 23 shows a color image forming apparatus.
This image forming apparatus comprises a charging roller 202, an unshown exposure device, a developing device 204 having developing units 204a, 204b, 204c and 204d, a cleaner 207a of the photosensitive drum 201 and an intermediate transfer drum 205 provided around the photosensitive drum 201, a cleaner 207b and a transfer roller 208 provided around the intermediate transfer drum 205 and a fixing unit 209 disposed a little apart from the intermediate transfer drum 205. Connected to the intermediate transfer drum 205 and the transfer roller 208 is a high-voltage power supply 210a. 
In forming a color image, first a photosensitive drum 201 is rotationally driven at a predetermined peripheral speed in the direction of the arrowhead, the surface of the photosensitive drum 201 is primarily charged by the charging roller 202, a laser beam 203 is scanning exposed by the unshown exposure device and a first color electrostatic latent image is formed on the photosensitive drum 201 and developed by the developing device 204.
The developing device 204 is equipped with a first, a second, a third and a fourth developing units 204a, 204b, 204c and 204d connectable and disconnectable from the photosensitive drum 201, in which a yellow toner, a magenta toner, a cyan toner and a black toner, for example, are contained respectively. The first latent image is developed by the first developing unit 204a and visualized as yellow toner image.
By applying a voltage of the polarity opposite to the charged polarity of the toner from the high-voltage power supply 210a to the intermediate transfer drum 205 in a first transfer position 206a where the intermediate transfer drum 205 and the photosensitive drum 201 contact with each other, the obtained yellow toner image is electrostatically transferred to the surface of the intermediate transfer drum 205 (primary transfer). The photosensitive drum 201 after the completion of primary transfer is subjected to the next color image forming after the removal of the toner remaining on the surface with the cleaner 207a. 
Similarly, after the primary charging of the photosensitive drum 201 by the charging roller 202, exposure with a laser beam 203 is made to form a second color electrostatic latent image and the latent image is developed by the second developing unit 204b to form a magenta toner image on the surface of the photosensitive drum 201. This magenta toner image is superposed and transferred over the yellow toner image on the intermediate transfer drum 205.
The above step is also repeated similarly concerning cyan and black color images, while the cyan and black toner images respectively obtained by the development of the third developing unit 204c and the fourth developing unit 204d are subsequently superposed over and transferred to the surface of the intermediate transfer drum 205. Thereby, a color image with four color toner images of yellow, magenta, cyan and black stacked is formed on the surface of the intermediate transfer drum 205.
Thereafter, the transfer roller 208 disconnected is brought into contact with the surface of the intermediate transfer drum 205 and a voltage of the polarity opposite the charged polarity of the toner is applied from the high-voltage power supply 210a to the transfer roller 208, so that at a second transfer position 206b, the four color toner image on the intermediate transfer drum 205 is collectively htransferred to the surface of a transfer material P to be conveyed at a predetermined timing to the second transfer position 206b where the intermediate transfer drum 205 and the transfer roller 208 contact each other (secondary transfer).
The transfer material P onto which the four color toner image is transferred is conveyed from the intermediate transfer drum 205 to the fixing unit 209, subjected to fixation with a heat roller or the like to make a permanent,image of full color, then discharged from the image forming apparatus outward. The intermediate transfer drum 205 after the completion of the secondary transfer is made ready for the next image forming, a certain amount of toner remaining on the surface is removed by the cleaner 207b to be set into action on the intermediate transfer drum 205 at a predetermined timing.
Besides, both of the image forming apparatuses mentioned above are so arranged as capable of forming images on both faces of a transfer material.
Especially in an image forming apparatus as shown in FIG. 22, however, since a transfer of superposing a toner image onto the transfer material is repeated, a so-called counter-transfer may occur in which the toner image transferred already is much apt to be transferred again to the photosensitive drum at the next transfer step.
In the case of forming a blue image, for example, first, a solid magenta image (image of maximum density set in the image forming apparatus) is formed on the transfer material, then a solid cyan image is multiply transferred on the solid magenta image and at a later transfer step, e.g., a transfer step of yellow or black, there takes place a situation that no toner to be transferred is present on the photosensitive drum. Under this situation, when a toner image on the transfer material comes near or contacts the surface of the photosensitive drum, a phenomenon may occur in which part of toner image (here, mainly cyan toner image) on the transfer material is very apt to be counter-transferred onto the photosensitive drum.
In this case, at the portion of cyan toner counter-transferred to the photosensitive drum, the density of the cyan toner image lowers and the color of magenta forming the image under the layer of the cyan toner image appears, thus resulting in a significant deterioration of image quality. Furthermore, this phenomenon is also a factor for making the stability of a multiple transfer difficult because of being dependent on a change in the toner charge amount or in the resistance value of a transfer material accompanying a change in temperature and humidity.
Besides, in recent years, since the particle diameter of toner has become smaller with a higher image quality, the toner charge amount per particle decreases but the total amount of charges tends to increase. Thus, the amount of transfer electric current or the transfer voltage required for the transfer of a solid image has increased. This rise in transfer electric current or transfer voltage is found to also adversely affect the above phenomenon of counter-transfer.
Besides, if the water content of a transfer material is small on occasions, an image fault (poor transfer) like marks of an abnormal discharge occurs during the progress of transfer.
Besides, in an image forming apparatus using an intermediate transfer member as shown in FIG. 23, a great amount of toner for four colors stacked on the intermediate transfer drum 205 ought to be transferred to the transfer material P at one stretch in the secondary transfer step as mentioned above. At this time, if the absolute charge amount of transferred toner is relatively small, the secondary transfer proceeds well.
As significantly noticeable in some used atmospheres of an image forming apparatus, especially under environments of low temperatures and low humidity, however, there was a problem that an image fault like marks of an abnormal discharge is very apt to occur when an attempt is made to transfer a full amount of toner from the intermediate transfer drum 205 to the transfer material P under a very large situation of absolute value of toner charge amount.
Since the toner charge amount for at least one color among the four colors of toner used in image forming is smaller than those of the other colors, after the toner for four colors is stacked on the intermediate transfer drum 205 to make the degree of transfer of the toner for four colors even, the toner on the intermediate transfer drum 205 is often subjected to recharging by means such as corona charging unit to equate the toner charge amounts for four colors, thus making the degree of each transfer even, but there were cases where the absolute value of toner charge amount became greater than before in this manner, thereby resulting in occurrence of an image fault like marks,of an abnormal discharge during the progress of the secondary transfer as with the above.
Besides, in both of the image forming apparatuses mentioned above image forming was made on the second face subsequent to image forming on the first face of a transfer material and at that time there were cases where a point-shaped loss of transfer occurred on the image transferred onto the second face, thus leading to the deterioration of image quality.
It is an object of the present invention to provide an image forming apparatus capable of preventing the image quality from deteriorating due to the occurrence of poor transferring at the transfer of an image to a transfer material.
The other objects of the present invention would be clarified by reading the following detailed description.